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872
FXUS63 KMPX 212351
AFDMPX

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
651 PM CDT Tue May 21 2024

.KEY MESSAGES...

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- Severe weather threat quickly diminishing this evening.

- Strong synoptic winds near 50 mph late this afternoon and
  evening in western Minnesota.

- Heavy rainfall is still expected, leading to an increased
  risk for river flooding going into the holiday weekend.

- Active pattern remains in place with several large systems
  bringing additional rain Friday and Memorial Day.
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&&

.UPDATE...
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Issued at 643 PM CDT Tue May 21 2024

With the line of storms having moved out of Minnesota into Wisconsin
the threat for severe weather in Minnesota has diminished greatly.
Rain and lightning will continue with the primary threat being urban
and small stream flooding. For the early evening the line will move
through western Wisconsin with high winds and an isolated
tornado as the threats. Once this line passes the remaining risk
in Wisconsin will be the same as Minnesota with rain and
lightning.
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&&

.DISCUSSION...
Issued at 350 PM CDT Tue May 21 2024

TODAY AND TONIGHT...Severe weather is still on track for this
afternoon and evening across much of south central Minnesota
and western Wisconsin. At the surface as of 18z, there are two
low pressure centers; one in far southwest Minnesota and a
second, stronger one near Omaha. The Minnesota low and its
associated warm front have lifted north into central Minnesota
this afternoon and has resulted in the warm sector making it
north of the Twin Cities metro. Previously, there was a lot of
uncertainty regarding how far north this would extend and thus
uncertainty with thunderstorm coverage on the northern and
western edge of this complex system. Further south with the
second, stronger low, a line of severe thunderstorms is
currently pushing into western Iowa. This activity should
continue to move east/northeast at a pretty good clip through
mid afternoon and eventually reach southern Minnesota by around
20z. With the clearing and strong advection of moist unstable
air across south central Minnesota, these storms should be able
to sustain themselves through western Wisconsin this evening.
CAMs and other guidance continue to highlight three main threats
with this event: the risk for all modes of severe weather,
heavy rain and flooding, and very strong synoptic winds on the
backside of the low.

For the severe risk...as mentioned above, all modes of severe
weather are expected this afternoon into this evening with the
highest threat across southern Minnesota and west central Wisconsin.
This is highlighted in the SPC SWODY1 discussion as well as PDS
Tornado Watch 277. Thunderstorm wind gusts in excess of 80 MPH,
isolated very large hail up to 4 inches, and strong (EF2+) tornadoes
are possible. A special 20z MPX sounding will hopefully give us more
insight to the environment, but CAMs and forecast soundings continue
to show very high shear/helicity and moderate MUCAPE values (1000-
1500 J/kg) with higher values 2000 J/kg + in extreme southeastern
Minnesota/Iowa/western Wisconsin. UH tracks have been fairly
consistent in moving across far southeastern Minnesota into western
Wisconsin with a second area further to the northwest across the
Twin Cities metro through 00z.

For the heavy rain/flash flood threat...CAMs are showing impressive
rainfall rates of 1-2"/hour with the storms as they move through.
Per the MPX 12z sounding, PWs were around 1.2" but have likely
increased to near climatological max. The other concern is the
training of storms and recent rainfall across southern Minnesota
over the past few days. This will likely create an elevated flash
flood threat. Additionally, the Twin Cities metro will be
susceptible to flash flooding given the potential for impressive
rainfall rates. This could coincide with the evening rush hour and
will need to be monitored closely.

For the strong wind threat...As the two previously mentioned lows
merge and deepen a little bit more by this evening, 45-50kt synoptic
wind gusts are possible across western Minnesota. A Wind Advisory
remains in effect for this region through 04z. These winds will
diminish below advisory criteria as the low pulls away to the west,
but gusts of 30-35kts are likely across central Minnesota (including
the metro) through 07-08z. Elevated northwesterly winds will remain
in place through Wednesday morning.

WEDNESDAY THROUGH MONDAY...High pressure will briefly settle in
for Wednesday and Thursday as the departing storm system lifts
to the northeast. A few showers will develop in the afternoon
along and north of I-94. Thursday will by dry, with northwest
winds becoming more southerly ahead of the next system that will
bring widespread rain Thursday night into Friday. Convergence
along this boundary is quite impressive, with southerly winds
south of it, and northeast winds on the poleward side. Did
increase precipitation chances toward the higher end of
guidance, as it looks like nearly all locations will see rain.

There will be a brief break in this system, but then another
shortwave trough will move across the region over the second
half of Memorial Day weekend.

&&

.AVIATION /00Z TAFS THROUGH 00Z THURSDAY/...
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Issued at 643 PM CDT Tue May 21 2024

Thunderstorms will continue to end from west to east this
evening. Northwest winds will develop and continue across the
region throughout the TAF period. VFR conditions will develop
overnight as ceilings clear. There is a chance for scattered
showers during the afternoon and evening at KAXN, KMSP, and
KSTC

KMSP...Steady rain and thunderstorms will continue for an hour,
but the thunder threat should diminish. Otherwise expect
northwest winds to increase overnight as skies clear and VFR
conditions settle in. The winds will become more westerly on
Wednesday and diminish.

/OUTLOOK FOR KMSP/
THU...VFR. Wind W 10 kts.
FRI...VFR/MVFR with -SHRA/TSRA likely. Wind SE 10-15 kts.
SAT...VFR. Wind SE 05-10 kts.
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&&

.HYDROLOGY...
Issued at 321 PM CDT Mon May 20 2024

Given the setup for widespread heavy rainfall, were certain to
see most rivers in the area end their recent recession and
begin rising once again. Extended probabilistic guidance (HEFS)
shows the potential for decent rises on area rivers should
rainfall exceed 2.5 inches or so over large areas, something
that is in the realm of possibility. The upper Minnesota and
tributaries, as well as the Crow river basin, are probably the
most susceptible to seeing rises to near or above flood stage by
later in the week should this occur. Urban/small
stream/overland flooding is also possible during and shortly
after some of the heavier downpours Tuesday and Tuesday night.

A reminder: The NWS hydrology/river forecast webpage (AHPS) at
water.weather.gov is going to be discontinued next week (May
28th). The new site (NWPS, the National Water Prediction
Service) is located at water.noaa.gov  now is the time to
replace bookmarks and familiarize yourself with how the new site
looks and feels. Resources for learning more are available at
the bottom of the this page:
https:/www.weather.gov/owp/operations

&&

.MPX WATCHES/WARNINGS/ADVISORIES...
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MN...Wind Advisory until 11 PM CDT this evening for Chippewa-
     Douglas-Lac Qui Parle-Pope-Redwood-Stevens-Swift-Yellow
     Medicine.
     Flood Watch until 1 AM CDT Wednesday for Anoka-Benton-Carver-
     Chisago-Dakota-Hennepin-Isanti-Kanabec-McLeod-Meeker-
     Mille Lacs-Morrison-Ramsey-Scott-Sherburne-Sibley-
     Washington-Wright.
     Flood Watch until 10 PM CDT this evening for Blue Earth-
     Faribault-Freeborn-Goodhue-Le Sueur-Martin-Nicollet-Rice-
     Steele-Waseca-Watonwan.
WI...Flood Watch until 1 AM CDT Wednesday for Pierce-Polk-St. Croix.
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&&

$$

UPDATE...NDC
DISCUSSION...Dye/JRB
AVIATION...JRB
HYDROLOGY...CCS